The present invention also relates to a shoe manufactured with such sole.
It is known that the footwear market is continuously evolving in order to seek and identify technical solutions that ensure optimum comfort for the end-user of the shoe.
As is well known, the comfort of a shoe depends not only on a correctly anatomical fit but also on correct outward permeation of the water vapor generated inside the shoe due to perspiration, in order to avoid the so-called “wet foot” phenomenon.
However, this water vapor permeation must not compromise the waterproofness of the shoe, and therefore solutions have been studied which entrust permeation to the upper or to the sole.
Most of the perspiration of the foot is produced at the interface between the sole of the foot and the sole of the shoe, and it is evident that the sweat that forms there is unable to evaporate and therefore condenses on the insole on which the foot rests. Only a minimal fraction of the sweat evaporates through the upper.
This problem is particularly important in shoes that have a plastic sole; in these cases, permeation through the sole is completely prevented (in the case of leather soles there is instead a small amount of permeation).
Solutions to the problem are provided by breathable and waterproof soles, which accordingly allow permeation of the sweat generated at the sole of the foot.
One of these solutions is disclosed in U.S. Pat. No. 5,044,096 and in EP-0382904 and consists in dividing the plastic sole into two layers with through holes and in interposing a waterproof breathable membrane (for example made of a material such as Gore-Tex® or the like), which is joined perimetrically and hermetically to the two layers, so as to allow no infiltrations of water.
This solution ensures correct permeation as well as an effective exchange of heat and water vapor between the environment inside the shoe and the outside environment, at the same time ensuring the necessary impermeability with respect to external moisture and water.
These perforated soles provided with waterproof and breathable membranes have certainly constituted a considerable innovation with respect to what was previously available.
Nonetheless, there are still aspects that can be improved, particularly in relation to the area occupied by the holes.
As is evident, the larger the total hole area, the greater the breathability; however, on the other hand, the number of holes provided in the tread and their diameter must be limited in order to prevent pointed foreign objects from entering through the holes and penetrating until they damage or pierce the membrane, which is delicate, since in practice it is a film and lacks adequate structural characteristics.
Such membrane is in fact continuously subjected to the compression performed by the foot, and therefore even a body that is not particularly pointed that penetrates one of the holes may cause damage without excessive difficulty.
One solution that has been adopted is to use a breathable protective layer, such as a felt, between the tread and the membrane.
Moreover, dirt, dust and pebbles may wedge in the holes of the tread, obstructing them and thus limiting breathability.
A different solution with respect to the use of a waterproof and breathable membrane lacking structural characteristics is disclosed in U.S. Pat. No. 6,508,015.
This patent discloses a sole that is provided by a structure with two layers, respectively an elastic upper layer, which is permeable to water vapor, and a lower layer, which covers less than 70% of the upper layer, which also acts as a support and as a tread.
The permeation activity of the sole is ensured by the microporous structure of the upper layer and by the shape of the lower layer.
The microporous structure of the upper layer is provided for example by means of sintered plastic material or by means of woven or non-woven structures made of synthetic material.
However, this layer does not have strictly waterproof characteristics; for this purpose, the patent mentions the possibility of rendering this layer hydrophobic, for example by treating the sintered polyethylene in high or ultra-high molecular weight conditions.
Another possibility for the waterproofing disclosed in the patent is to add, above the upper layer, an additional layer formed by a waterproof membrane.
Although this described solution solves the problem of the breathable area of the sole, which is large, it does not adequately meet the requirement of waterproofing said sole.
It has in fact been found that the hydrophobic treatment of the sintered material does not make the upper layer sufficiently waterproof, especially in the case of large amounts of water.
Moreover, the idea of coupling an impermeable membrane to the inner layer is not in itself sufficient to ensure perfect insulation from water, since infiltrations of water along the perimeter of the upper layer are possible.
Another problem that is linked to this type of sole is that the upper layer tends in any case to absorb considerable amounts of water (“sponge effect”), which is released over time, leading to evident soiling of the surfaces on which one walks.
This problem becomes more evident as the size of the pores of the material increases.
Already for pore dimensions of more than 5 μm, there is penetration of unclean water (dirty or soapy water): in this case, the surface tension is lower than the typical value of water (73 mN/mm).